Owing to the low thermal mass of such elements and their generally low vaporization temperature, it is necessary to protect them from overheating in the event of incorrect use of appliances to which they are fitted or malfunction of the element itself.
Conventionally, a mineral insulated element is protected by an electromechanical device such as a domed bi-stable, bi-metallic blade which is arranged so that it adopts a stable position in contact with a part of the element and thereby retains a switch in the electrical supply circuit to the element in a position which maintains the electrical supply. However, should the temperature of the element rise above a predetermined threshold temperature which is above the normal operating range, then the blade will move into its other stable position and cause the switch to operate to cut off the electrical supply to the element. As soon as the temperature of the blade drops below the threshold temperature then it will revert back to its original stable position to enable the electrical supply to be once more restored to the element.
As a back-up to the blade in the event that it should fail to function correctly, part of the device is made of a fusible or thermoplastic material which is designed to melt or to soften if a second predetermined threshold temperature higher than the aforesaid first temperature is reached. This is intended to cause the switch to disconnect and thereby permanently cut off the electrical supply to the element.
However, as thick film resistive heating elements have a low thermal mass, the rate of rise of temperature under fault conditions is so high that it is not sufficient simply to arrange an electromechanical control device as described above in contact with such an element in the same way as with a mineral insulated element to protect it from damage and to ensure that it will operate efficiently.
It is, therefore, an object of the present invention to provide a thick film resistive heating element which is adapted for use with a conventional electromechanical controller similar to the type described above.